The present invention relates generally to the field of NOx emissions control in fossil fuel-burning boilers, and, in particular, to a new and useful method of reducing NOx emissions by fuel reburning with oxygen.
The combustion of fossil fuels generates oxides of nitrogen, such as NO and NO2, cumulatively referred to as NOx. Emissions of NOx in the atmosphere are increasingly becoming a health and environmental concern. The U.S. Environmental Protection Agency (EPA) has determined that regulation of NOx emissions is necessary and appropriate, thereby creating an urgent need to develop more efficient NOx emissions control technologies.
In a conventional fossil fuel combustor, combustion air and a fossil fuel are mixed and provided to a main flame zone within a furnace. NOx, a byproduct of the combustion, is formed when naturally occurring nitrogen in the fuel and/or molecular nitrogen in the combustion air oxidize.
Fuel reburning is a technology capable of reducing NOx emissions. The technology includes providing an oxygen-deficient secondary combustion reburn zone above an oxygen-rich main combustion zone. Supplementary fuel provided to the reburn zone generates hydrocarbon radicals, amines, and cyanic species that react with incoming main combustion products to convert NOx to N2. Additional air may then be provided by overfire air (OFA) ports, placed above the reburn zone, to burn combust the remaining fuel and combustible gases.
Fuel reburn applications generally utilize flue gas recirculation (FGR) technology to reduce NOx emission. Flue gas from downstream of boiler is recirculated via conduits back to the secondary combustion zone as an oxygen-lean carrier gas, thereby maintaining a fuel-rich environment and enhancing the fuel penetration and mixing with the main combustion zone gases and products. Quenching, resulting from utilizing flue gas from downstream of the boiler outlet as a carrier gas, further inhibits NOx formation in the reburn zone.